aoa. i am going to upgrade my pc. i have 2 options for processor one is
intel pentium 4 3.06 HT with 1MB CACHE
and second is
intel pentium D dual core 2.6GHz with 2MB CACHE. AND motherboard is intel D865GSA.
now please suggest me which processor is better. i want to upgrade for linux based server and also for gamming.

i don't know about the motherboard, but i'd recommend the pentium d processor. the d has two proper processors, the ht makes one processor core appear as two processors, but the overall performance boost with ht is a lot lower. while the pentium d runs at a lower speed (2.6ghz), it does more work per cycle than the pentium 4. also, the larger cache will help with server workloads.

for similar reasons, you should get centrino processors instead of pentium 4 or pentium 4-m processors for laptops -- larger cache and more work per cycle. the new core duo and core 2 duo processors are based on similar technology to the centrino.

if you can wait a few months, i'd recommend getting the desktop core 2 duo processors. the pentium d is being phased out.

unrelated: the cache is why certain processors are better for servers. for example, this server from ibm. 1.9mb L2 cache, and 34mb L3. they have other models with 70+mb L3 cache. i believe some of the hppa processors (from hp, used for hp-ux) had up to 3mb L1 and 64mb L2 cache. the itanium 2 processors have up to 24mb L3.

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I have a question...if you have a dual core pentium D, and windows
xp pro 2002. will the two cores be enabled by default or there
has to be a patch installed or SP2 installed.
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once you have pentium D, with lots of ram, and everything looking
good...The ATA hard disk can be a bottle neck.

IMO SATA 2 is the right choice if you really want performance from
your dual core system.